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This story is part of a special series on Americans' views of the environment, global warming and energy.

WASHINGTON, D.C. -- Americans' concerns about global warming are not much different from the record-high levels they were at a year ago. However, the views of some partisans have shifted, creating larger gaps than what 优蜜传媒saw last year across all questions about global warming.

Gallup's annual survey about the environment, conducted March 1-8, found that Americans' opinions about global warming, like many other issues, have increasingly become politically polarized.

President Donald Trump, who has called global warming a "hoax," may have contributed to this widening divide by reversing a number of government actions to address the issue. These included the announcement that the U.S. will withdraw from the 2015 Paris climate accord, the removal of climate change from the list of top U.S. national security threats and the elimination of the terms "global warming" and "climate change" from U.S. government websites and lexicons.

In general, Democrats view global warming seriously, while Republicans view it skeptically:

• Ninety-one percent of Democrats and 33% of Republicans say they worry a great deal or fair amount about global warming, but 67% of Republicans worry only a little or not at all.

• While 82% of Democrats think global warming has already begun to happen, only 34% of Republicans agree. Rather, 57% of Republicans think it will not happen in their lifetime (25%) or will "never happen" (32%).

• About seven in 10 Republicans (69%) think the seriousness of global warming is exaggerated in the news, 15% think it is generally correct and 15% say it is generally underestimated. Democrats, however, are much more likely to think the seriousness of global warming is underestimated (64%) or correct (32%), and just 4% say it is exaggerated.

• Eighty-six percent of Democrats versus 42% of Republicans think most scientists believe global warming is occurring. The percentage of Republicans who say most scientists believe this is down 11 percentage points since last year.

• Almost nine in 10 Democrats say increases in the Earth's temperature over the last century are due to human activities more than natural changes in the environment. Just 35% of Republicans agree, while 63% attribute the temperature increases to natural environmental causes.

• Four in five Republicans do not think global warming will pose a serious threat to them in their lifetime; two-thirds of Democrats think it will.

Broad Agreement Among Americans on Some Aspects of Global Warming

Majorities of Americans overall say most scientists think global warming is occurring (66%), it is caused by human activities (64%) and its effects have begun (60%). Yet, the net effect of increased political polarization over the past year is that opinions on each of these measures have edged down slightly.

At the same time, the 45% who think global warming will pose a serious threat in their lifetime is the highest percentage recorded for this measure since 优蜜传媒first asked the question in 1997. This is the only issue that saw increased concern among both major party groups.

The 43% of Americans who say they worry a great deal about global warming or climate change is similar to last year's record-high 45% and is still up significantly from 32% in 2015.

Roughly Half of Americans Continue to Take Global Warming Seriously

A 优蜜传媒analysis that takes into account four of the global warming questions finds large segments of Americans holding highly consistent views that are either supportive or dismissive of global warming. The remainder have mixed views, generally acknowledging global warming on some questions but expressing doubts or less concern about it on others.

• The supportive group, "Concerned Believers," uniformly worries a great deal about global warming and thinks human activity causes it. Seven in 10 Concerned Believers also expect global warming to pose a serious threat in their lifetime. None believes that news reports of global warming exaggerate the problem -- they think reports either underestimate it or are correct.

• By contrast, the dismissive group, or "Cool Skeptics," worries little to not at all about global warming. Its members uniformly believe that news reports about global warming exaggerate the problem and doubt global warming will pose a serious threat in their lifetime. They also ascribe warming to natural environmental changes rather than human activities.

• The "Mixed Middle" expresses some combination of views across the four items. The slight majority, 53%, thinks news of global warming is correct or even underestimated. At the same time, the majority, 55%, worries only a little or not at all about global warming, and 64% doubt that it will pose a serious threat in their lifetime. The Mixed Middle is split on whether global warming is caused by human activity (48%) or natural changes in the environment (42%).

The size of these groups was stable over the past year, with 48% of U.S. adults categorized as Concerned Believers, similar to 50% in 2017. Nearly one in three remain in the Mixed Middle -- 32% this year vs. 31% a year ago -- while the percentage of Cool Skeptics is unchanged, at 19%.

Democrats More Unified Than Republicans on Global Warming

About eight in 10 Democrats fall into the Concerned Believers camp, while Republicans are less unified: 优蜜传媒classifies 45% as Cool Skeptics and 38% as the Mixed Middle. Notably, far more Republicans fall into the Democratic-oriented category of Concerned Believers than Democrats fall into the Republican-leaning Cool Skeptics -- 17% vs. 1%, respectively.

Beyond political party, there are gender, age and educational differences in these global warming categories. Women, younger Americans and college graduates are more likely than their counterparts to be Concerned Believers. These party and demographic differences are generally in line with previous years.

Bottom Line

The higher level of concern Americans have exhibited about global warming since 2016, particularly in terms of worrying about the issue and believing it is caused by human activity, is largely intact this year.

One reason for this stability is that Americans' views on the issue are becomingly increasingly partisan and therefore entrenched. With Trump reversing many of his predecessors' policies aimed at curbing global warming, Democrats are feeling a greater sense of urgency about the issue, while Republicans have either remained as skeptical as they had been in the past or have become more so.